Radially anisotropic magnet body

ABSTRACT

A method of manufacturing an annular, radially anisotropic, permanently magnetizable body substantially free of cracks and having an inner diameter which is at least 0.8 times the outer diameter and consists of a material of the formula MeO.6Fe 2  O 3 , Me being barium, strontium or lead in which after a molded from said material in a radial magnetic field, the body is hydrostatically compressed to increase its density prior to sintering.

This is a division of application Ser. No. 507,366, filed Sept. 18,1974, now abandoned.

The invention relates to a curved, sintered, radially anisotropic,permanent magnetisable body essentially consisting of a ferrite of theformula MeO.6Fe₂ O₃, wherein Me is at least one of the metals barium,strontium and lead. The term radially anisotropic is to be understood tomean that the body is magnetisable much more easily in the radialdirection than in other directions. The said ferrites have a hexagonalcrystal structure and show an easy axis of magnetisation in thedirection of the hexagonal axis. Small particles of said ferrites have aplate shape perpendicular to the hexagonal axis. Permanent magnetisablebodies are built up from magnetically elementary particles which in thiscase, as said above, are plate-shaped. Since the bodies are radiallyanisotropic, the particles are present in the body with their hexagonalaxis substantially in the radial direction. For a high remanence of thebody a high and preferably a uniform density is necessary. Moreover, fora high remanence in the radial direction an orientation of the particleswhich is as well as possible in the radial direction is necessary.

In practice, segment-like bodies having a center angle up to 135° areused in the case of such curved bodies. Since the segments areanisotropic in the radial direction, inhomogeneities in density andorientation have been introduced during the manufacture thereof, namelyduring the formation of the molded body which is sintered afterwards.The inhomogeneities and the structural anisotropy in the molded bodycause considerable deformations of the segment during sintering. Thesintered body should be given the desired shape, which is done bymachining. Said machining results in an increase of cost in themanufacture of the segments. Moreover, the inhomogeneities in densityand orientation have a detrimental influence on the magnetic quality andthe strength of the segment.

As regards curved bodies one is restricted in practice to segments,because curved, sintered, radially anisotropic bodies of a differentshape always show cracks and are therefore not useful. It has now beenfound that said cracks are a result of mechanical stresses which arebuilt up during the cooling because the coefficients of thermalexpansion in the radial direction and in the tangential direction aredifferent. Said stresses can easily exceed the strength of the material.

According to the invention the body has a closed annular shape with aninside diameter which is at least 0.8 times the outside diameter. In aring having such a diameter ratio the largest stresses after cooling aresmaller than the strength of the material. The rings thus do not showcracks. With a diameter ratio smaller than 0.8 the sintered body is nolonger a closed ring but it is broken into a number of pieces or thesintered body shows radial or tangential cracks which are not acceptablefor practical purposes.

In manufacturing anisotropic permanent magnetisable bodies the formationof the molded body occurs in a magnetic field which orients theparticles of the ferrite. In the case of a radially anisotropic body aradial magnetic field is used. Since a homogeneous radial magnetic fieldis difficult to realise it is to be preferred to give the moulded body,prior to sintering, an after-treatment which suppresses the occurrenceof unfavourable thermal stress distributions during the heating of themoulded body. The invention therefore also relates to a method ofmanufacturing such a body in which a powder of the ferrite is formedinto a molded body in a radial magnetic field and is sintered, and inwhich the density of the material of the molded body is first increasedby hydrostatic compression.

The FIGURE in the drawing shows an annular magnetic body consisting ofBaFe₁₂ O₁₉. The outside diameter is 35.1 mm and the inside diameter is29.8 mm. The ratio between them is 0.85. the body has a density of 4.9g/cm. The remanence in radial direction is 4100 Gauss.

Starting from a finely ground powder of BaFe₁₂ O₁₉, an annular moldedbody having an outside diameter of 32.2 mm and an inside diameter of31.0 mm was manufactured in a radial magnetic field. After havingprovided the molded body in a rubber bag it was recompressed with ahydrostatic pressure of 1000 kg/cm². It was then heated in air at a rateof 160° C./hour to 1250° C., which temperature was maintained for twohours after which it was cooled to room temperature at a rate of 40°C./hour.

What is claimed is:
 1. A method of manufacturing an annular, radiallyanisotropic, permanently magnetizable body having an inner diameterwhich is at least 0.8 times the outer diameter substantially free ofcracks and consisting essentially of a material of the formula MeO0.6Fe₂O₃, wherein Me is at least one of the metals barium, strontium and leadcomprising the steps of molding a powder consisting of said materialinto an annular body having an internal diameter at least 0.8 times theouter diameter while in a radial magnetic field, hydrostaticallycompression said body to increase the density thereof, thereafterheating said body to a temperature of about 1250° C. to form ahighly-coherent body, and cooling said body to ambient temperature atabout 40° C./hour.